This invention relates to the fabrication of jointed structures for operation under cryogenic conditions, and more particularly to extending a threaded stud above a laminated composite material so that another structure can be fastened to the stud.
The problem to which the invention is directed arose in the context of fabrication of propellant tanks for space vehicles. In spacecraft, the launch weight is of primary importance, as every ounce of excess weight which can be removed from fixed structure can be used for storage of propellant, to thereby provide a longer operational lifetime. Thus, all parts of a space vehicle and its launcher are subject to intense efforts to reduce weight. The propellant storage tanks are prime subjects for weight reduction, as they tend to be among the largest structures on the spacecraft andor its launcher. Past efforts at weight reduction of propellant tanks have led to innovations such as the use of laminated composite materials, made from multiple layers of strong, light fibers, such as graphite or carbon fiber, impregnated with a polymer. Many techniques are known for fabricating such tanks, as described, for example, in U.S. Pat. No. 5,427,334, issued Jun. 27, 1995 in the name of Rauscher, Jr., and U.S. Pat. No. 5,441,219, issued Aug. 15, 1995 in the name of Rauscher, Jr., which deal with fabrication of composite tanks having integral structures such as pipes and flow control devices.
A major problem with the application of composite materials to the storage of propellants in a spacecraft context is that the propellants are often cryogenic fluids, and some of the fluids, such as hydrogen, have very small molecules. The composite materials tend to have relatively large coefficients of thermal expansion in directions perpendicular to the reinforcing plies, so that relatively large changes in dimensions of the composite structure tend to occur when propellant is introduced into the tank, and then when the propellant is withdrawn. In the case of relatively small tanks, the plumbing required to carry the propellant to the engine may be made integral with the tank, as described by Rauscher, Jr., but large tanks require removable structures, such as hatches for ingress and egress, and flange attachments for attaching large-diameter pipes associated with large propellant flux.
FIG. 1a is a simplified plan view of the upper portion 10 of a prior-art propellant tank having an outer surface 10os, showing a central aperture 12 surrounded by a ring 14 of threaded apertures, some of which are marked 14a, 14b, and 14c. Both aperture 12 and the ring 14 are concentric with an axis 8. The central aperture 12 may be for any purpose, such as ingress or egress of personnel, or for flow of fuel. FIG. 1b is a perspective or isometric view of a portion of a propellant pipe 16 with a flange 18 which defines a ring 20 of apertures 20a, 20b adapted to fit over, and register with, the threaded apertures of ring 14, for accommodating bolts for connecting pipe 16 to the aperture 12 of upper tank portion 10 of FIG. 1a. For completeness of understanding, bolts 22a and 22b of a set of 22 of bolts is illustrated adjacent flange 18. Each bolt 22a, 22b, . . . of set 22 of bolts is threaded to match the threads of one of the threaded apertures 14a, 14b, . . . of ring 14 of threaded apertures.
FIG. 2a is a cross-section of upper tank portion 10 in a region near threaded aperture 14a of FIG. 1a. In FIG. 2a, the outer surface 10os is at the top of the FIGURE, and the inside surface 10is is at the bottom. The thickness of the laminated composite material is selected to be as thin as possible for weight reduction, but thick enough to withstand the forces associated with the mass of the propellant being stored, the anticipated acceleration, the safety factor, and possibly other factors or considerations. Among these other considerations is that of preventing leakage of propellant directly through the composite material. Considering that a propellant may have molecules as small as hydrogen, which is notorious for its ability to leak through the most minute apertures, the thickness of the laminated composite must be adequate to reduce leakage to an acceptable level. As illustrated in FIG. 2a, the threaded aperture 14a includes threads 214t defined in a metallic insert 214. Metallic insert 214 is fastened into a portion of a cylindrical aperture 215 centered on a local axis 208, either adhesively, by a force-fit, or both. The threaded end of bolt 22a extends into the threads 214t. An additional aperture portion 215a extends below the cylindrical aperture 215, for accommodating a bolt length which, when torqued, extends below threaded insert 214. It has been discovered that the tension applied to the insert 214 during torquing of the bolt 22a when fastening flange 18 of FIG. 1b to tank outer surface 10os has the potential to cause delamination, as illustrated by delamination cavity 220 in FIG. 2b. Such delamination can also result from the use of a bolt which contacts the bottom portion 215b of aperture 215 during torquing. In addition, delamination can result from lateral forces applied to the head end of a bolt threaded into the insert 214. Thus, there are many possible causes of delamination. Delamination, by its nature, is not well controlled, since it literally involves disintegration, or the breaking up of an integral or monolithic material, at least in a local region. The damage associated with delamination may extend toward the inner surface, and compromise the ability of the tank to contain propellant.
The large tanks which are fabricated to carry propellant for a launch vehicle or space vehicle are expensive items. Delamination damage to the apertures 215 of FIG. 2a, as illustrated in FIG. 2b, must be repaired in a suitable manner, or the entire tank discarded. These repairs are rendered difficult by the need to seal against egress of the cryogenic propellant regardless of the changes in dimension of the laminated composite due to its coefficient of thermal expansion as it makes the transition between room temperature (or above) and cryogenic temperatures.
Improved composite tank fabrication and repair are desired.
A method according to an aspect of the invention is for fastening a threaded stud to project above a first surface of a composite laminate in a substantially leakproof manner as to cryogenic liquid gases. The laminated composite structure is undesirably subject to crushing above a predetermined pressure and delamination under excessive tension perpendicular to the plies. The method comprises the step of forming a through aperture through the composite material at the location at which the stud is to be installed. The through aperture should have a first diameter adjacent the first surface of the composite laminate, a frustoconical surface in the form of the frustum of a cone lying adjacent a second surface of the composite laminate, and a second diameter, smaller than the first diameter, in a region lying between the frustoconical surface and the portion of the aperture having the first diameter. An internally threaded insert is installed from the first side into the aperture so as to fasten the insert within the portion of the aperture having the first diameter. A bolt is obtained which includes a head, a nonthreaded shank portion adjacent the head, and a threaded portion remote from the head. The threaded portion of the shank should mate with the internal threads of the insert. The bolt is made from a material having a known coefficient of thermal expansion which is less than or lower than the coefficient of thermal expansion of the laminated structure. A generally cylindrical collar is obtained. The collar should be made from a material having a particular coefficient of thermal expansion and a thickness or length in an axial direction. In one particular embodiment of the invention, the coefficient of thermal expansion of the collar is ideally near zero. The collar so obtained should have a bore about its axis no greater in diameter than the diameter of the nonthreaded shank of the bolt, and the collar should also have an overall diameter about the axis which is no less than the diameter of the cone adjacent the second surface of the laminated composite. The collar should further have a planar first end surface orthogonal to the axis, and a second end surface. The second end surface should include a peripheral planar annulus parallel with the surface of the first end, and also include a depressed frustoconical portion in the shape of the frustum of a cone depressed below the planar annulus. According to an aspect of the method, a first seal, which in a preferred embodiment of the invention is a xe2x80x9ckxe2x80x9d seal, is assembled onto the bolt, with the conical or frustoconical portion of the first k seal facing away from the head of the bolt. The collar (or a plurality of such collars) is then assembled onto the bolt, with the depressed portion of the collar facing the first k seal. A second seal, preferably a k seal, is placed on the bolt, with the conical portion of the second k seal facing away from the first surface of the collar, to thereby generate an assembled bolt. The assembled bolt is inserted through the aperture from the second side of the laminated composite, and threaded through the insert, so as to cause a portion of the threaded portion of the bolt to protrude from the first side of the laminated composite. In that case in which the coefficient of thermal expansion of the collar is selected in conjunction with the coefficient of thermal expansion of the bolt and with the thickness of the laminated composite in such a manner that the pressure applied to the laminated composite by the bolt does not change substantially over the expected temperature range, the bolt is torqued into the insert to achieve a pressure in the laminated composite which is less than the pressure at which the composite laminate crushes. The torque should be sufficient to deform the k (or other) seals sufficiently to seal against the pressure of the propellant.
In a particular mode of the method, the aperture in the laminated composite is formed by drilling from one of the first and second sides, and the laminated composite is supported from the other one of the first and second sides to aid in preventing delamination during the drilling. In another mode, the drilling is accomplished at the slowest drill feed rate at which reasonable cutting occurs, in order to reduce drill thrust loading. In another mode, the frustoconical portion of the aperture is formed to a 120E included angle, and the collar includes a conical bore portion with a 120E total included angle. The depressed conical portions are preferably polished. In one version, a countersunk region surrounds the conical portion of the aperture. Adhesive is preferably placed on the threads before the bolt is threaded into the insert. Before the bolt is torqued, it may be desirable to clean at least the frustoconical portion of one of the aperture and a corresponding conical portion of the second k seal. The adhesive, if any, should be cleaned from exposed portions of the threads.
A structure according to an aspect of the invention includes a laminated composite with a threaded stud extending from a first side thereof. The structure includes a through aperture in the laminated composite. The through aperture defines an axis, and includes a first portion adjacent the first side of the laminated composite. An internally threaded insert is fastened in the first portion of the through aperture. The through aperture further includes a second portion adjacent a second side of the laminated composite. The second portion has a frustoconical surface (a surface in the form of a portion of a cone) depressed below the second surface of the laminated composite. The structure includes an annular collar defining an axial bore coaxial with the axis, a first end surface perpendicular to the axis, and a second end defining a planar annulus centered on the axis. The collar also defines a frustoconical surface depressed below the second end annulus. A bolt includes a head, a nonthreaded shank adjacent the head, and a threaded portion remote from the head. The bolt is assembled with the laminated composite coaxial with the aperture, the collar, and two k seals having frustoconical surfaces. The parts are assembled so that (a) is the first k seal lies between the head and the second end of the collar, with the frustoconical portion of the first k seal mated with the depressed frustoconical surface of the collar, (b) the second k seal lies between the first end surface of the collar and the frustoconical surface of the second portion of the through aperture, with the frustoconical portion of the second k seal mated with the frustoconical surface of the second portion of the through aperture, and (c) the threaded portion of the bolt threaded through the internally threaded insert sufficiently to extend above the first surface of the laminated composite.
In a particular version of the structure, the material of the laminated composite has a coefficient of thermal expansion which exceeds that of the material of the bolt, so that the pressure exerted by the bolt on the laminated composite will tend to increase with increasing temperature. In this embodiment, the collar(s) is (a) made from a material, and (b) has a length selected (i) in conjunction with the length of the bolt lying within the laminated composite, (ii) the material of the bolt, and (iii) the material of the laminated composite, so that the pressure applied to the laminated composite by the combination of the bolt and the collar tends to remain constant with temperature. In one embodiment, the collar(s) may have a coefficient of thermal expansion near zero. In this particular version of the structure, the bolt is torqued sufficiently to apply pressure to the laminated composite which is less than that pressure at which the laminated composite crushes. The pressure will remain more or less constant over the temperature range from room temperature to cryogenic temperatures.
A further avatar of the structure comprises adhesive lying in the interstice between the internal threads of the insert and the threads of the bolt. In a preferred embodiment, the unthreaded portion of the shank of the bolt is polished. Also, at least one of the depressed frustoconical surface of the collar and the depressed frustoconical surface of the aperture is polished. A region surrounding the frustoconical surface of the aperture may be countersunk below the second surface of the laminated composite.
In a further avatar of the invention, a bolted structure includes a containment barrier defining a containment vessel suitable for use with cryogenic fluids. The containment barrier further defines first and second surfaces, and a through aperture extending between the first and second surfaces. The through aperture is dimensioned to clear the shank of a bolt. A bolt is included. The bolt includes a head, an unthreaded shank portion, and a threaded shank portion. The bolt extends through the through aperture from one of the first and second surfaces in such a manner that at least the threaded portion of the bolt protrudes past the other one of the first and second surfaces. The bolted structure also includes a CTE collar including a bore having a diameter selected to clear the shank of the bolt. The bore of the collar is penetrated by at least a portion of the shank of the bolt. A nut is threaded onto at least a portion of the threaded portion of the bolt, and the nut is torqued relative to the bolt to provide a predetermined tension in the shank of the bolt. The bolted structure also includes one of
(a) a seal interposed between, and in immediate contact with, the head of the bolt and that portion of the one of the first and second surfaces of the barrier which meets sealing standards for the seal;
(b) a seal interposed between, and in immediate contact with, the nut and that portion of the one of the first and second surfaces which meets sealing standards for the seal; and
(c) a first seal interposed between an end surface of the collar and that portion of one of the first and second surfaces which meets sealing standards for the seal; and
a second seal interposed between a second end surface of the collar and a surface portion of another structure.